A standard elevator has a shaft vertically traversing a plurality of different floors of a building and a cabin or car guided on vertical rails in the shaft. A cable has one end connected to the top of the car and another end connected to a counterweight, and is looped over a pulley mounted at the top of the shaft. A motor typically mounted in a room that is provided on the building roof is connected to this pulley, so it can rotate it in one direction to raise the elevator and sink the counterweight, and in the opposite direction to lower the elevator and raise the counterweight. This counterweight weighs about as much as the car plus its average load, although for various reasons it can weigh more or less.
Such a structure requires that a substantial room be devoted to the cumbersome drive. Since the motor must be able to move the entire high-inertia mass of the elevator, the counterweight, and any passengers or freight, it must be fairly powerful, so it is fairly large. When provided on the building roof this motor room uses normally otherwise unused space. However, in an existing structure it is frequently not possible to mount it on the roof, as the building is not designed for such a load. Providing the drive at the base of the shaft can solve this problem, although developing such underground space is normally very difficult and expensive.
Hence recourse has been had to zero-headroom elevators which carry their own drives, so that the elevator shaft need not extend much above or below the floors being served. Such an elevator can be installed in an existing structure fairly easily.
The typical such system guides the car on four rails formed with straight-across teeth, that is teeth whose flanks are formed by families of lines that are all parallel and perpendicular to the longitudinal direction of the rails. These rails are engaged by respective complementarily toothed sprockets carried by respective drive motors. Even when assembled to high tolerances, such an arrangement wears rapidly, as the four sprockets inherently work somewhat differentially on the rails. Simple thermal expansion of the system can cause it to bind and wear excessively, and other factors can desynchronize the sprockets just as easily. Once such an arrangement has worn a little, the rate of wear increases rapidly.
Another arrangement has only two toothed rails that are engaged by respective sprockets driven by respective worms sitting on the opposite ends of a motor shaft. To facilitate meshing, the sprockets have teeth each of whose one flank is inclined corresponding to the pitch of the worm and whose other flank is straight, that is parallel to the radius. Such teeth are unfortunately weaker one one side of the sprocket than on the other. The teeth of the rail are complementarily formed as a row of recesses, that is they are not laterally open, and the teeth of one face of the rail are narrower than the other rail face. Such teeth cannot be produced easily, and are very difficult to machine to close tolerances. Hence such a system also wears rapidly.
It is also known for the two rails to have confronting faces formed as racks with straight-across teeth engaged by respective complementary sprockets driven by the same motor. Each rail has a pair of oppositely directed guide grooves lying in a vertical plane perpendicular to the plane of the rails. Four respective angle irons carried on the car have flanges projecting vertically into these grooves, thereby accurately guiding the car on the rails. Such an arrangement can bind readily and will wear rapidly when not in perfect alignment.
In fact such zero-clearance elevators have all proven so troublesome that they are no longer made or used on any meaningful scale.